1. Field of the Invention
The present invention relates to a Group III nitride compound semiconductor device and, more particularly, to a Group III nitride compound semiconductor device which functions as a light-emitting device.
2. Background Art
A Group III nitride compound semiconductor is one type of semiconductor used among direct transition types of semiconductors, which have an emission spectrum widely ranging from ultraviolet to red. Thus, the semiconductor is employed in light-emitting devices such as light-emitting diodes (LEDs) or laser diodes (LDs). In a device fabricated by stacking Group III nitride compound semiconductor layers, a sapphire substrate is typically employed as a substrate in vie (because) of its proximity in lattice constant. FIGS. 3 and 4 show structures of the semiconductor devices.
FIG. 4 shows a conventional laser diode (LD) 950 employing Group III compound semiconductors. In the LD, an AlN buffer layer 912, an n-GaN n-contact layer 913, an n-AlxGa1xe2x88x92xN n-cladding layer 914, an n-GaN n-guide layer 915, an emission layer 916 formed, preferably of a multi-layer (multiple quantum well, MQW), a p-GaN p-guide layer 917, a p-AlxGa1xe2x88x92xN p-cladding layer 918, and a p-GaN p-contact layer 919 are formed on a sapphire substrate 911, in the order presented. A positive electrode 920A is formed on the p-contact layer 919 while a negative electrode 920B, developed through etching, is formed on a portion of the n-contact layer 913.
However, the aforementioned conventional semiconductor has a drawback. Specifically, when a Group III nitride compound semiconductor is formed on a sapphire substrate, cracks are generated in a semiconductor layer, or a semiconductor layer bends, since elastic modulus and thermal expansion coefficient of the sapphire substrate differ from those of the Group III nitride compound semiconductor. Thus, the fabricated device has poor device characteristics. In addition, although the lattice constant of the sapphire substrate is approximately equal to that of the Group III nitride compound semiconductor, dislocations are readily generated due to misfit. Particularly, a cladding layer formed of AlxGa1xe2x88x92xN attains a higher elastic modulus as the compositional proportion of Al (hereinafter simply referred to as xe2x80x9cxxe2x80x9d) increases. Therefore, cracks are readily generated in such a cladding layer during a cooling process in production of a semiconductor device. As a result, the thickness of the cladding layer, which has a large compositional proportion of Al is limited to a low value. Such limitation in thickness is particularly detrimental to fabrication of laser diodes.
Employment of a sapphire substrate, which is an electric insulator, raises another limitation for fabricating semiconductor devices. Specifically, a positive electrode and a negative electrode must be disposed on a semiconductor-formed surface of a sapphire substrate.
In view of the foregoing disadvantages, an object of the present invention is to provide a Group III nitride compound semiconductor device in which generation of cracks in a semiconductor layer, bending of a semiconductor layer, and generation of misfit-induced dislocation in a semiconductor layer are prevented. Another object of the invention is to provide a Group III nitride compound semiconductor device employing a conductive substrate through which electricity is passed.
Accordingly, the present invention is directed to a Group III nitride compound semiconductor device comprising a substrate and one or more Group III nitride compound semiconductor layers formed on a first surface or first and second surface of the substrate, wherein aluminum gallium nitride (AlxGa1xe2x88x92xN, 0 less than x less than 1) is employed as the substrate.
Preferably, among the Group III nitride compound semiconductor layers stacked on a first surface or first and second surfaces of a substrate, all layers, having a thickness of more than 10 nm, are independently formed of a compound represented by AlxGa1xe2x88x92xN, wherein 0xe2x89xa6xxe2x89xa61.
Preferably, each of the Group III nitride compound semiconductor layers stacked on a first surface or first and second surfaces of a substrate is formed of a compound represented by AlxGa1xe2x88x92xN, wherein 0xe2x89xa6xxe2x89xa61.
Preferably, a first layer of a Group III nitride compound semiconductor layer stacked on a first surface or a first and second surface of a substrate has a thickness of 1-20 xcexcm, more preferably 2-20 xcexcm.
A group III nitride compound semiconductor layer comprises binary compounds such as AlN, GaN, and InN. A group III nitride compound semiconductor layer also comprises ternary compounds such as AlxGa1xe2x88x92xN, AlxIn1xe2x88x92xN, and GaxIn1xe2x88x92xN (0 less than x less than 1). And a group III nitride compound semiconductor layer further comprises quaternary compounds such as AlxGayIn1xe2x88x92xxe2x88x92yN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61). In the present invention, unless otherwise specified, the term xe2x80x9cGroup III nitride compound semiconductorsxe2x80x9d encompasses Group III nitride compound semiconductors per se and Group III nitride compound semiconductors doped with an impurity which causes the semiconductors to become either p- or n-conduction type semiconductors. Likewise, aluminum gallium nitride (AlxGa1xe2x88x92xN, 0 less than x less than 1) also encompasses doped semiconductors.